1. Field of the Invention
The present invention generally relates to high-speed binary communications. More specifically, the invention relates to a novel apparatus and method which reduces Inter-Symbol Interference (ISI) under variable cabling environments within the SCSI protocol.
2. Description of Related Art
Computers are generally constructed with at least one parallel data bus capable of transferring digital information between a host computer processor and hardware devices such as printers, scanners, disk drives, etc. FIG. 1 illustrates a parallel bus 100 connected to a host 120 and various hardware devices 140a . . . 140m. The number of hardware devices 140a . . . 140m that can communicate via parallel bus 100 is limited by the type of electronic interface, i.e., bus, used in the computer's design.
One well-known electronic interface that is employed in the architecture of computers is the Small Computer System Interface (SCSI). SCSI is a set of ever changing electronic interface standards that allow faster and more flexible parallel communication between computers and peripheral hardware over previous interfaces. The maximum length of cable that can comprise the bus as well as the maximum number of devices that can be connected to it is dictated by the specific SCSI standard employed within the computer. For example, the SCSI-1 standard specifies a bus that can be six meters in length and can interconnect up to eight devices. A list of the currently adopted SCSI standards and their main attributes are summarized in TABLE 1.
TABLE 1MaximumMaximumCableMaximum SpeedNumberSCSI StandardLength (m)(Mbytes/sec)of DevicesSCSI-16 5 8SCSI-26 5–108 or 16Fast SCSI-2310–20 8Wide SCSI-232016Fast Wide SCSI-232016Ultra SCSI-3, 8-bit1.520 8Ultra SCSI-3, 16-bit1.54016Ultra-2 SCSI1240 8Wide Ultra-2 SCSI128016Ultra-3 (Ultra160/m) SCSI12160 16
SCSI interfaces are susceptible to the same signal distortion interference that is seen in most digital transmission devices. As data transmission speeds and cable lengths increase, signals transmitted using SCSI interfaces become vulnerable to inter-symbol interference (ISI), which, in a digital transmission system, is distortion of the received signal. This distortion is exhibited by temporal spreading and consequent overlap of individual pulses to the degree that the receiver cannot reliably distinguish between change of states, i.e., between individual signal elements.
Contributing factors leading to inter-symbol interference include timing inaccuracies between pulse timing and the channel ringing; insufficient bandwidth; amplitude distortion as a function of frequency; phase distortion as a function of frequency; variations in loading conditions as a function of the number of devices connected to the cable; and length of the cable.
Regardless of the circumstances responsible, inter-symbol interference results in unreliable data detection when messages are passed between two or more devices in a SCSI environment. In an effort to combat the effects of ISI, several conventional equalization approaches have been adopted. In one approach, equalization of the digital pulse signal is accomplished by utilizing a second order zero (s2) in the frequency domain to narrow the pulse into a pre-defined target shape. Maximum likelihood detectors are then used to extract the relevant data. This approach reduces the amplitude distortion effect associated with ISI, but is practically useless against correcting phase distortion.
Another conventional approach employs a matched filter having a transfer function that is the inverse of the transmission media transfer function. This approach acts to neutralize both the amplitude and phase distortion induced by the cable. While this is an effective method for the reduction of ISI, it is impractical because of the multiple signal lines that are utilized with the SCSI communication link. In addition, the SCSI cabling environment must accommodate a wide variety of device configurations making this scheme difficult as well as economically infeasible to provide a single match filter that encompasses all cases.
An apparatus and method is needed, therefore, that can overcome the limitations of conventional approaches, thereby reducing ISI in a variable cable environment and making data detection reliable. Additionally, a need exists such that the implementation of the apparatus and method is relatively easy and cost effective.